This invention relates to a process for impregnating reinforcing fibers. More particularly, the invention relates to a solvent-free method for impregnating reinforcing fiber yarn or tow with a synthetic resin and to an apparatus for producing resin-impregnated tow.
Composites comprising reinforcing fiber imbedded in a resin matrix are well-known. Both organic and inorganic fiber may be used for making such composites, separately and in combination. Woven fabric and continuous fiber strand in the form of yarn, tow, roving, tape and even monofilament have utility in such applications, and a wide variety of analogous compositions based on chopped fiber are also used for producing molded goods. One of the common methods for making composites employs continuous fiber strand in the form of flat tape or ribbon, preimpregnated with a curable resin. The processes used currently for impregnating the fiber strand are generally wet processes that employ low viscosity liquid resins, or solutions and dispersions of more highly viscous liquid resins and solid polymers. Wet processes are relied upon in part because such methods tend to ensure a more thorough penetration of the fiber bundle. Whether the wet-impregnated strand is to be used immediately in a filament winding operation or stored in the form of prepreg for later fabrication, it is necessary that any volatile components such as residual solvent be removed, usually by application of heat and/or vacuum. The presence of even minor amounts of residual solvent or other volatiles in the prepreg tends to produce voids and similar defects during the subsequent curing step which threaten the integrity of the composite. Removing these volatiles without significantly advancing the degree of cure in a thermoset resin is difficult.
Melt-coating and extrusion-coating processes for applying molten resin to the surfaces of strand and filament are also well known, as typified by wire and cable coating processes, as well as by calendering operations for applying coatings for laminating webs such as those disclosed in U.S. Pat. No. 3,874,833. Coating processes have also been modified for impregnating use in filaments and strand. Generally thermoplastic resins which will not become thermally cured when heated are preferred for use in these processes. Methods and apparatus designs are also available for directly imbedding strand into a thermoplastic resin matrix, including those shown for example in U.S. Pat. No. 4,439,387. Fiber-reinforced structural shapes comprising fibers imbedded in a thermosetting resin matrix may be produced by pultrusion processes in which fiber and thermosetting liquid resin are combined and pulled through a die to give continuous shaped articles such as I-beams, channels, bars and rods. Curing is accomplished thermally during the shaping operation.
Recently, methods for impregnating fiber strand with thermosetting resins at a high rate with control of fiber resin content have become known. For example, in the process shown in U.S. Pat. No. 3,908,042, resin is applied sequentially to each side of a fiber strand by contacting rolls or kiss rolls carrying a resin film of controlled thickness. The roll is partly immersed in a bath of heated resin, acquiring a surface coat of resin as it rotates. The thickness of the resin film on the surface of the kiss roll, and thereby the amount of resin applied to the strand, is controlled by means of a doctor blade placed in contact with the roll ahead of the point of contact with the fiber strand. Conventional alternatives for controlling the amount of resin applied to a fiber strand have included the use of dies for wiping the excess resin from the strand as it exits a resin bath. See, for example, U.S. Pat. No. 4,211,818. Removing excess resin from the strand by a wiping operation is generally difficult, and made more difficult when strand formed of very fine, frequently brittle fiber such as carbon is used. Broken fibers, termed fuzzies or fuzz balls, tend to form and accumulate at the die face and break the fiber strand. Additionally, applying the resin either by means of a kiss roll or by immersion in a resin bath requires that a substantial quantity of resin be heated and maintained at an elevated temperature, increasing the likelihood that the resin will be advanced or cured. Premature curing impairs impregnation because of increased resin viscosity, and produces defective and unacceptable prepreg.
A practical method for impregnating reinforcing fiber yarn or tow with a resin uniformly and at a high rate is therefore still needed by the composites industry. Problems created by the use of solvents and by heating thermoset resins for extended periods as is currently practiced would be reduced or avoided. More accurate control over the resin content, the quality and the uniformity of the resulting prepreg would be a substantial advance in this art.